Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies.

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins.

Unconventional slowing down of electronic recovery in photoexcited charge-ordered LaSrFeO.

The coupling of ordered electronic phases with lattice, spin, and orbital degrees of freedom are of central interest in strongly correlated systems. Their interplay has been intensively studied from femtosecond to picosecond time scales, while their dynamics beyond nanoseconds are usually assumed to follow lattice cooling. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition.

Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets.

Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300 to 700 K using static and transient spectroscopies as well as in situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission line width with temperature elevation up to ∼500 K, losing a factor of ∼8 to 10 in PL intensity at 400 K relative to ambient.

Charge Carriers Modulate the Bonding of Semiconductor Nanoparticle Dopants As Revealed by Time-Resolved X-ray Spectroscopy.

Understanding the electronic structure of doped semiconductors is essential to realize advancements in electronics and in the rational design of nanoscale devices. Reported here are the results of time-resolved X-ray absorption studies on copper-doped cadmium sulfide nanoparticles that provide an explicit description of the electronic dynamics of the dopants. The interaction of a dopant ion and an excess charge carrier is unambiguously observed via monitoring the oxidation state.

Transient Melting and Recrystallization of Semiconductor Nanocrystals Under Multiple Electron-Hole Pair Excitation.

Ultrafast optical pump, X-ray diffraction probe experiments were performed on CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump fluence. Bragg peak shifts related to heating and peak amplitude reduction associated with lattice disordering are observed. For smaller NCs, melting initiates upon absorption of as few as ∼15 electron-hole pair excitations per NC on average (0.

Nanomaterial-based sensors for the detection of biological threat agents.

The danger posed by biological threat agents and the limitations of modern detection methods to rapidly identify them underpins the need for continued development of novel sensors. The application of nanomaterials to this problem in recent years has proven especially advantageous. By capitalizing on large surface/volume ratios, dispersability, beneficial physical and chemical properties, and unique nanoscale interactions, nanomaterial-based biosensors are being developed with sensitivity and accuracy that are starting to surpass traditional biothreat detection methods, yet do so with reduced sample volume, preparation time, and assay cost.

Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin.

Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 10(5)-10(6) m(-1) that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

Intracellular FRET-based probes: a review.

Probes that exploit Förster resonance energy transfer (FRET) in their feedback mechanism are touted for their sensitivity, robustness, and low background, and thanks to the exceptional distance dependence of the energy transfer process, they provide a means of probing lengthscales well below the resolution of light. These attributes make FRET-based probes superbly suited to an intracellular environment, and recent developments in biofunctionalization and expansion of imaging capabilities have put them at the forefront of intracellular studies. Here, we present an overview of the engineering and execution of a variety of recent intracellular FRET probes, highlighting the diversity of this class of materials and the breadth of application they have found in the intracellular environment.

Electric Field Modulation of Semiconductor Quantum Dot Photoluminescence: Insights Into the Design of Robust Voltage-Sensitive Cellular Imaging Probes.

The intrinsic properties of quantum dots (QDs) and the growing ability to interface them controllably with living cells has far-reaching potential applications in probing cellular processes such as membrane action potential. We demonstrate that an electric field typical of those found in neuronal membranes results in suppression of the QD photoluminescence (PL) and, for the first time, that QD PL is able to track the action potential profile of a firing neuron with millisecond time resolution. This effect is shown to be connected with electric-field-driven QD ionization and consequent QD PL quenching, in contradiction with conventional wisdom that suppression of the QD PL is attributable to the quantum confined Stark effect.

Fast, Ratiometric FRET from Quantum Dot Conjugated Stabilized Single Chain Variable Fragments for Quantitative Botulinum Neurotoxin Sensing.

Botulinum neurotoxin (BoNT) presents a significant hazard under numerous realistic scenarios. The standard detection scheme for this fast-acting toxin is a lab-based mouse lethality assay that is sensitive and specific, but slow (∼2 days) and requires expert administration. As such, numerous efforts have aimed to decrease analysis time and reduce complexity.

Synthesis and Ligand Exchange of Thiol-Capped Silicon Nanocrystals.

Hydride-terminated silicon (Si) nanocrystals were capped with dodecanethiol by a thermally promoted thiolation reaction. Under an inert atmosphere, the thiol-capped nanocrystals exhibit photoluminescence (PL) properties similar to those of alkene-capped Si nanocrystals, including size-tunable emission wavelength, relatively high quantum yields (>10%), and long radiative lifetimes (26-280 μs). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy confirmed that the ligands attach to the nanocrystal surface via covalent Si-S bonds.

Picosecond energy transfer and multiexciton transfer outpaces Auger recombination in binary CdSe nanoplatelet solids.

Fluorescence resonance energy transfer (FRET) enables photosynthetic light harvesting, wavelength downconversion in light-emitting diodes (LEDs), and optical biosensing schemes. The rate and efficiency of this donor to acceptor transfer of excitation between chromophores dictates the utility of FRET and can unlock new device operation motifs including quantum-funnel solar cells, non-contact chromophore pumping from a proximal LED, and markedly reduced gain thresholds. However, the fastest reported FRET time constants involving spherical quantum dots (0.

Silicon nanocrystals at elevated temperatures: retention of photoluminescence and diamond silicon to β-silicon carbide phase transition.

We report the photoluminescence (PL) properties of colloidal Si nanocrystals (NCs) up to 800 K and observe PL retention on par with core/shell structures of other compositions. These alkane-terminated Si NCs even emit at temperatures well above previously reported melting points for oxide-embedded particles. Using selected area electron diffraction (SAED), powder X-ray diffraction (XRD), liquid drop theory, and molecular dynamics (MD) simulations, we show that melting does not play a role at the temperatures explored experimentally in PL, and we observe a phase change to β-SiC in the presence of an electron beam.

Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS Core/shell nanocrystals: utilizing IV/II-VI semiconductor epitaxy.

Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots.

In situ optical and structural studies on photoluminesence quenching in CdSe/CdS/Au heterostructures.

We report here detailed in situ studies of nucleation and growth of Au on CdSe/CdS nanorods using synchrotron SAXS technique and time-resolved spectroscopy. We examine structural and optical properties of CdSe/CdS/Au heterostructures formed under UV illumination. We compare the results for CdSe/CdS/Au heterostructures with the results of control experiments on CdSe/CdS nanorods exposed to gold precursor under conditions when no such heterostructures are formed (no UV illumination).

Thermal stability of colloidal InP nanocrystals: small inorganic ligands boost high-temperature photoluminescence.

We examine the stability of excitons in quantum-confined InP nanocrystals as a function of temperature elevation up to 800 K. Through the use of static and time-resolved spectroscopy, we find that small inorganic capping ligands substantially improve the temperature dependent photoluminescence quantum yield relative to native organic ligands and perform similarly to a wide band gap inorganic shell. For this composition, we identify the primary exciton loss mechanism as electron trapping through a combination of transient absorption and transient photoluminescence measurements.

Tetraalkylammonium uranyl isothiocyanates.

Three tetraalkylammonium uranyl isothiocyanates, [(CH(3))(4)N](3)UO(2)(NCS)(5) (1), [(C(2)H(5))(4)N](3)UO(2)(NCS)(5) (2), and [(C(3)H(7))(4)N](3)UO(2)(NCS)(5) (3), have been synthesized from aqueous solution and their structures determined by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO(2)(NCS)(5)(3-), and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds 1-3, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate.

Uranyl sensitization of samarium(III) luminescence in a two-dimensional coordination polymer.

Heterometallic carboxyphosphonates UO(2)(2+)/Ln(3+) have been prepared from the hydrothermal reaction of uranyl nitrate, lanthanide nitrate (Ln = Sm, Tb, Er, Yb), and phosphonoacetic acid (H(3)PPA). Compound 1, (UO(2))(2)(PPA)(HPPA)(2)Sm(H(2)O)·2H(2)O (1) adopts a two-dimensional structure in which the UO(2)(2+) metal ions bind exclusively to the phosphonate moiety, whereas the Ln(3+) ions are coordinated by both phosphonate and carboxylate functionalities. Luminescence studies of 1 show very bright visible and near-IR samarium(III)-centered emission upon direct excitation of the uranyl moiety.

Hydrothermal synthesis of uranyl squarates and squarate-oxalates: hydrolysis trends and in situ oxalate formation.

We report the synthesis of two uranyl squarates and two mixed-ligand uranyl squarate-oxalates from aqueous solutions under hydrothermal conditions. These products exhibit a range of uranyl building units from squarates with monomers in (UO(2))(2)(C(4)O(4))(5).6NH(4).

Capturing hydrolysis products in the solid state: effects of pH on uranyl squarates under ambient conditions.

Two uranyl squarates, (UO(2))(6)(C(4)O(4))(3)(OH)(6)O(2)·9H(2)O·4NH(4) (1; a = 16.6897(7) Å, cubic, I23) and (UO(2))(C(4)O(4))(OH)(2)·2NH(4) (2; a = 8.5151(4), b = 15.